Insulin secretion is controlled because:
beta cells have potassium and calcium ion channels to maintain potential difference of -70 mV
as glucose concentration increases, glucose enters beta cells
respiration of glucose produces ATP. ATP gated potassium ion channels close so potassium ions no longer diffuse out of cell.
Potential difference in cell becomes more positive which leads to depolarisation.
calcium ion channels open which triggers exocytosis of insulin.